The trend toward smaller microelectronic devices, such as those with sub-micron geometries, has resulted in devices with multiple metallization layers to handle the higher densities of circuit elements. One common metal used for forming metal lines on a semiconductor wafer, also referred to as wiring, is aluminum. Aluminum has the advantage of being relatively inexpensive, having low resistivity, and being relatively easy to etch. Aluminum has also been used to form interconnections in vias to connect the different metal layers. However, as the size of via/contact holes shrinks to the sub-micron region, a step coverage problem emerges, which in turn can cause reliability problems when using aluminum to form the interconnections between the different metal layers. Such poor step coverage results in high current density and enhances electromigration.
One approach to providing improved interconnection paths in the vias is to form completely filled plugs by using metals such as tungsten, while using aluminum for the metal layers. However, tungsten processes are expensive and complicated, tungsten has high resistivity, and tungsten plugs are susceptible to voids and form poor interfaces with the wiring layers.
Therefore, copper has been proposed as a replacement material for interconnect metallizations. Copper has the advantages of improved electrical properties as compared to tungsten, as well as better electromigration and lower resistivity as compared to aluminum. The drawbacks to copper are that it is more difficult to etch as compared to aluminum and tungsten, and it has a tendency to migrate into the dielectric layer, such as silicon dioxide. To prevent such migration, a barrier layer, such as titanium nitride, tantalum nitride and the like, must be used prior to the depositing of a copper layer.
Typical techniques for applying a copper layer, such as electrochemical deposition, are generally only suitable for applying copper to an electrically conductive layer. Thus, an underlying conductive seed layer, typically a metal seed layer such as copper, is generally applied to the barrier layer prior to electrochemically depositing copper. Such seed layers may be applied by a variety of methods, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD). Typically, seed layers are thin in comparison to other metal layers, such as from 5 nm to 150 nm thick.
While the copper seed layer improves the adhesion of the electroplated copper layer to the barrier layer, it also introduces a surface upon which an oxide can grow prior to electroplating the main copper layer. Such an oxide forms from exposure of the metal seed layer to oxygen, such as air. The longer the seed layer is exposed to oxygen, the greater the amount of oxide formation.
It was originally believed that any oxide that formed on the surface of the copper seed layer interfered with subsequent copper electroplating. For example it was believed that any oxide layer on the copper seed layer would cause voids to form in the electroplated copper layer. Interestingly enough, it has recently been determined that native oxide layers formed from air are much more detrimental to the electroplated copper layer than oxides formed from pure oxygen. Actually, it has presently been observed that oxides formed from pure oxygen in fact benefit the subsequent copper electroplating. For instance, a copper seed layer having a thin, but pure, oxide layer formed thereon wets better than a copper seed layer having no oxide layer at all. Unfortunately, the process currently used to introduce the pure oxide layer onto the copper seed layer also introduces a thin native oxide into the process.
Accordingly, what is needed in the art is a method for electroplating copper that introduces an ultra-pure oxide layer over the copper seed layer without introducing a native oxide layer therebetween.